**4. Validation**

The validation of CFD simulations is of great importance and is defined by the American Institute of Aeronautics and Astronautics [79] as "The process of determining the degree to which a model is an accurate representation of the real world from the perspective of the intended uses of the model." In order to perform a validation, different experimental investigations can be carried out. It should be noted that the experiments are also not free of measurement errors, which must also be taken into account in the statistical statement. Depending on the target process variable, different methods are suitable for validating different bioreactor simulations. One of the most fundamental factors is the flow pattern. Particle image velocimetry (PIV) [80– 82], laser doppler anemometry [83, 84] and laser induced fluorescence [81, 85] are suitable for measuring the velocities in a bioreactor and are especially used for stirred bioreactors. For orbitally shaken [86, 87] and wave-mixed [88] systems, often only the liquid height or distribution is measured.

If the power input is of relevance, it is reasonable to use it as a validation criterion. Thus, the measurement of the torque around the rotation axis is suitable for stirred [74, 89], orbitally shaken [90, 91] and wave-mixed [92] bioreactors. Alternatively, the specific power input can be determined either by measuring the electric current supplied to the motor [93, 94] or calorimetrically [95, 96]. Villiger et al. [97] described a method, which allows the maximum hydrodynamic stress to be determined using poly(methyl methacrylate) aggregates [98], which is also suitable for validation purposes. In addition, the mixing time can also be used as a validation parameter.

*Computational Fluid Dynamics for Advanced Characterisation of Bioreactors Used… DOI: http://dx.doi.org/10.5772/intechopen.109848*

Depending on whether the mixing time is evaluated globally or locally in the CFD simulation, the decolourisation method or the conductivity measurement according to DECHEMA e.V. Working Group for Single-Use Technology are both suitable for validation [19]. However, these are not the only valid approaches, for example, Vivek et al. [99] used a Raman spectroscopy probe to experimentally determine the mixing time to validate CFD-determined mixing times.

For aerated systems, either the *k*L*a* value can be used directly as a validation parameter, or the bubble size distribution can be used. The *k*L*a* value can be measured using different methods described in Seidel and Eibl [8] or Garcia-Ochoa and Gomez [100], and the bubble size distrbution by shadowgraphy [101], optical multimode online probe [102], capillary suction probe technique [103, 104], SOPAT endoscopy [105, 106] or focused beam reflectance [107, 108] measurements. The gas holdup, which is typically determined optically, by the differential pressure method [109] or radar level gauge [93] is also suitable for aerated systems.

For suspension studies with microcarriers or other particles, it is advisable to use the suspension behaviour or the *N*S1 criterion for validation. For this purpose, the particles that are to be suspended are added to an unstirred bioreactor. Then the speed is increased stepwise until the *N*S1 criterion is achieved. The evaluation is either performed by eye [110] or recorded with a camera [59, 111–113]. Delafosse et al. [113] and Loubière et al. [59] have refined this evaluation method by measuring the light attenuation using a light source mounted on one side of the bioreactor and the camera on the opposite side. Most authors have recorded lateral images as well as images from below, using a mirror for the latter. For the validation of their CFD simulations, Zhang et al. [114] recorded images with a high-speed camera. However, they did not examine cells grown on microcarriers, but rather immobilised *Lactobacillus* cells.
